Electrodeless lamps can provide advantages over electrode lamps. The electrodeless lamps require no electrical connections, can be energized without direct mechanical contact to the lamps, and can be energized by the field action of remote radio frequency, optical stimulation, or even magnetic fields. Instead of using electrical current passing through electrodes to excite an electrodeless lamp for producing light, in most cases radio frequency energy is induced through a quartz glass envelope to excite the gas in the lamp and cause it to emit photonic radiation. Primarily used in ultraviolet curing applications where power and speed are requirements, this lamp technology offers significant benefits in other applications as well.
Electrodeless lamps can be run at much higher power levels than electrode lamps allowing them to produce much greater amounts of ultraviolet light than their electrode counterparts. Electrodeless lamps exhibit long life spans on the order of 20,000 hours and, theoretically, can last much longer than that. They are very sturdy and withstand both mechanical and thermal shock and vibration very well.
Electrodeless lamps provide engineering flexibility. Lamp geometries are not fixed in size and shape, and can easily be adjusted to conform to the needs of difficult applications. Among these are applications such as treatment with ultraviolet light in polymer curing operations and in water treatment. Though in the past electrodeless lamps have not generally been used in water purification systems, they can be much better than electrode lamps for this purpose.
In some respects, industry is heavily invested and dependent on using electrode lamps. Because of this, electrodeless lamps have not been used as extensively as they would otherwise have been. The key reasons for this are technical. Electrodeless ultraviolet applications require more sophistication and finesse to engineer than do electrode models. Among the most difficult challenges in using electrodeless lamps is engineering a method for exciting and controlling the output of the lamps. In most cases radio frequency power and coupling systems are used to power the lamps. Lamp geometries, and fill mixes, which are the combination of elements that are excited by an energy source to make ultraviolet light, are engineered to couple with the lamps. In many applications the coupling is achieved, but control of the lamp becomes difficult due to dependence of the coupling on the temperature of the lamps, and the lamps are prone to thermal runaway.
Another problem is that, without special envelope material, in many applications electrodeless lamps produce large amounts of ozone. Ozone can be hazardous to man and machine and should be tightly managed.
Among these problems the chief reason that electrodeless lamps are not used more is that they are extremely difficult to manage and control. In radio frequency applications as an electrodeless lamp continues to operate, it couples more and more strongly with and draws more and more energy from the available radio frequency field, which in turn makes it increase its operating temperature. Subsequently, that causes it to couple more strongly, and it draws more of the available energy. Although this runaway results in more relative ultraviolet output, it also causes the peak wavelength output of the lamp to change because the peak wavelength output of the lamp is dependent on the operating temperature of the lamp. This causes the lamp to be less useful for some applications.
For example, lamps filled with a gas mixture comprising mercury gas and argon gas, the most common fill mix, have not been widely used for water purification because the germicidal bandwidth needed for water purification occurs at about 240 nm (nanometers or 10−9 meters) to 265 nm wavelength. The problem is that emission of photons at this wavelength range occurs best when the lamp is kept in a temperature range of from about 90 F (degrees Fahrenheit) to 110 F. Thermal runaway causes the lamp to undesirably exceed this temperature, causing the desired wavelengths to fall off, while other wavelengths, such as those used in some kinds of curing rise dramatically. The peak emission wavelength usually rises to about 360 nm. Such a wavelength is good for curing some kinds of polymer compositions but is not good for killing water borne bacteria. This lack of lamp output stability at the germicidal wavelengths has prevented this technology from being developed for various uses requiring specific output wavelengths. This is true for uses such as water purification, and a method for controlling the characteristic thermal runaway is needed.